This invention relates to thermoset epoxy foam composition of delta-1-tetrahydrophthalic anhydride and epoxy resin and a method of preparing them.
The use of acid anhydrides as curing agents for epoxy resin is known in the art. Examples of acid anhydrides that have been used are maleic, citraconic, itaconic, pyromellitic, dodecylsuccinic, phthalic and hydrogenated phthalic anhydrides like tetra and hexahydrophthalic anhydrides. Of the hydrogenated phthalic anhydrides, the tetrahydrophthalic anhydride (THPA) is less preferred than the lower melting hexahydrophthalic anhydride (HHPA).
The THPA is used in admixture with HHPA in a ratio of 13 parts of THPA with 78 parts of HHPA of 25 parts of THPA with 50 parts of HHPA and 25 parts of phthalic anhydride. also, THPA is used as a liquid mixture of THPA isomers, delta-1-THPA, delta-2-THPA, delta-3-THPA and delta-4-THPA. These THPA isomers differ in the position of the double bond in the closed ring portion of the molecule. Independently, each isomer is a solid at room temperature but when the isomers are mixed together the resulting mixture is a liquid at room temperature. These isomers are also referred to as cyclohexene-1,2-dicarboxylic anhydride isomers with the prefix 1, 2, 3 or 4 to indicate the position of the double bond. The most readily available isomer is the delta-4-THPA or 4-cyclohexene-1,2-dicarboxylic anhydride.
When THPA is used as a curing agent for an epoxy resin in the above described manner to produce a foamed epoxy product, a blowing agent must be introduced into the THPA and epxoy resin mixture. The blowing agent liberates a gas or vapor at the curing temperature of the THPA and epoxy resin mixture before gelation occurs. Typical blowing agents are high boiling gases, such as Freon gases; or low boiling liquids, which vaporize at the curing temperature to release a gas. Examples of the decomposition type blowing agents are Celogen [p,p'-oxybis-(benzenesulfonylhydrazide)] an Unicel ND (dinitrosopentamethylenetetramine).
Some curing agents react in such a manner as to liberate the gas by themselves, while other curing agents may be reacted with certain compounds to produce the gas for foaming. "Handbook of Epoxy Resins" by Lee and Neville, McGraw-Hill Book Company, New York, 1967, provides examples of these types of curing agents. Examples of the former type are the amine boranes, aconitic acid and substituted phosphitoborohydrides. Examples of the latter are the addition of N,N'-dinitroso-N,N'-dimethyl terephthalamide to fatty-polyamide curing agents to liberate nitrogen or the addition of hydrogen peroxide to primary amine curing agents. Another example of the former type of curing agent according to U.S. Pat. No. 3,320,187, Samuel L. Burt, is amine carbamate salts which decompose to produce carbon dioxide. An article by Michelotte, Knuth, and Bauley, J. Chem. Eng. Data 4, 79, 1959 gives other examples of the above described latter type of curing agent. They disclose that curing agents, citraconic, itaconic, and chlorocitraconic anhydrides release carbon dioxide when reacted with benzyldimethylamine, a tertiary amine. They suspect that the carbon dioxide is released by the decarboxylation of the named anhydrides. The named anhydrides in the presence of base lose an allylic hydrogen and undergo a self-condensation of unknown nature, eliminating carbon dioxide in the process. A foamed epoxy produced in this manner with these curing agents has a dark color.
Some of the curing discussed above are also used to produce syntactic foams. Epoxy foams of the syntactic type, syntactic foams, are produced by adding low-density fillers into the formulations of epoxy resin and curing agent. Foams of this type involve the utilization of microscopically small hollow spheres made of phenolic resin, clay, urea formaldehyde, glass, or polystyrene. These spheres contain gas or liquid that cause the spheres to expand or balloon when heated or that act as blowing agents when the spheres are broken by vibration.
Epoxy foams have cells which have either an open cell structure or a closed cell structure or various amounts of open cells and closed cells. The closed cell structures have low thermal conductivity and low water absorption, whereas open cell structures have better acoustic insulation properties and cushioning characteristics. The general principle is that flexible materials tend to have open cell structures while rigid materials tend to have closed cell structures, but there are exceptions. The type of cell structure is largely determined by the method of expansion. The epoxy foams made by thermal decomposition of chemical blowing agents are usually foams with a high percentate of closed cells. Epoxy foams prepared by the "in-situ" chemical reactions can be either the open cell or closed cell type. Another foaming parameter is cell size, which is approximately 0.1-5 mm for foams below 0.10 pcf (pounds per cubic foot) and 0.5-5 mm for foams greater than 10 pcf. Cell shape is dependent on foaming techniques; such as, open mold or floating platen.
Illustrative of the prior art pertinent to the production of epoxy foams are the following U.S. Pat. Nos.: 3,282,863 (Carey et al.); 3,310,507 (Shepherd); 3,630,990 (Hazen et al.) and 3,341,555 (Wooster et al.).
It is the object of this invention to prepare thermoset epoxy foam compositions which are either epoxy chemical foams or epoxy syntactic foams by using delta-1-tetrahydrophthalic anhydride as both a curing agent and a foaming agent.